A major problem with flowing a fluid through a conduit, such as a duct or pipe, is the pressure losses which accumulate over the distance travelled by the fluid. A principal source of the losses is two-dimensional boundary layer separation which occurs immediately downstream of sharp turns in the conduit.
Separation is a result of the lack of momentum in the boundary layer of the flow in the new direction dictated by the turn in the conduit. This causes the boundary layer along the surface of the inside corner of the conduit to detach from the surface immediately downstream of the turn. The fluid adjacent to the conduit surface in the separation region flows in the reverse direction, due to the inability of the momentum of the flow to overcome the back pressure in the flow, and interaction with the flow of the bulk fluid produces an eddy which recirculates the fluid. The recirculation removes energy from the flow and results in a pressure loss proportional to the size of the separation region.
Another problem associated with the separation region is the pressure pulses generated in the flow as the re-attachment point of the bulk fluid fluctuates in position. The re-attachment position is the downstream point where the separation region ends and the flow of the bulk fluid contacts the surface again. The position of the re-attachment point fluctuates as the size of the separation region varies and larger separation regions produce larger pressure pulses. The generation of the pressure pulses increase the instability of the flow and can damage, or increase the noise level associated with, components actuated by the flow.
One method to overcome the loss in fluid pressure is to increase the pressure of the fluid at the inlet of the conduit by an amount equal to the accumulated pressure losses. This solution is undesirable due to the added cost of producing a higher inlet pressure and of fabricating a conduit around the increased pressure requirements. Additionally, this solution would generate larger separation regions and larger pressure pulses. Another solution is to route the flow such that turns are kept to a minimum. Although a conduit without any turns would be ideal, for many purposes a straight conduit is impractical. It is, therefore, highly desirable to conduct a flow of fluid through an angled conduit with minimal pressure losses.